Feb 25 (Reuters) - A Nigerian tribunal will rule on Tuesday whether the election of President Umaru Yar'Adua was valid, a decision that could entrench a disputed government or tip Africa's most populous nation into uncertainty.

The following are key facts about the tribunal.

* WHAT IS THE TRIBUNAL?

It is a panel of five judges set up to handle legal challenges to Yar'Adua's poll victory last April by his two closest opponents, former army ruler Muhammadu Buhari and former Vice-President Atiku Abubakar.

* WHAT CAN IT DO?

The tribunal can either uphold Yar'Adua's victory or order a re-run of the election.

* IS ITS DECISION FINAL?

No, the losing parties can appeal and the legal battle could last several more months. Buhari and Abubakar are likely to appeal if they lose. Yar'Adua has not said what he would do if the judgment went against him.

* WHAT IS THE EVIDENCE?

Buhari and Abubakar presented hundreds of kilos of documents, including ballot papers and result sheets which they said were falsified by the electoral commission. They have also presented a report by European Union observers who said the election was "not credible" because of widespread fraud and violence.

* WHAT IS AT STAKE?

The leadership of Africa's most populous nation with 140 million people. Nigeria is also the world's eighth-biggest exporter of crude oil, producing 2.1 million barrels per day.

* WHAT HAPPENS IF YAR'ADUA LOSES?

If he appeals, he will remain president until the Supreme Court makes a final decision. If he opts not to appeal, or if he loses his appeal, he will hand over to the Senate president for a maximum of 90 days during which a new election must be held. However, the tribunal has some discretion in interpreting the law and may give its own instructions.

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(Reporting by Estelle Shirbon; Editing by Matthew Tostevin)

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